The present invention relates to an impeller for a turbo pump provided with a volute casing or a diffusertype casing, for a water jet propulsion machinery mainly used as propulsion means for ships. The invention also relates to a turbo pump including the impeller.
Since a water jet propulsion system for propulsion means for ships, in which a turbo pump is used, has been hitherto regarded as lower in the efficiency of propulsion than a propulsion system using a propeller, it has not been generally used. Only for safety reasons, the water jet propulsion system is used to propel a smaller sized ship for leisure for which the efficiency of propulsion is not so much important. Under existing circumstances, as a result of theoretical studies, trial manufacture and experiment, it has merely turned out that the propulsion efficiency of the water jet propulsion system can be made higher than that of a conventional propeller propulsion system in use so far as a high-speed ship is concerned.
A turbo pump in the water jet system is used in an arrangement as shown in FIG. 1. In operation, water is sucked in through an intake port A, increased in pressure by the pump, and discharged in the form of jet at a speed V.sub.j from a nozzle B, to thereby propel the ship by the reaction to the discharge. The characteristic of the jet from the nozzle B is determined depending on the cross-sectional area of the nozzle, and is shown by a curve J in FIG. 2 which is a graph with the axis of abscissa for the flow rate Q of jet and the axis of ordinate for the pressure energy (water head) H.
The characteristics of the water jet propulsion system when the ship shown in FIG. 1 is travelling, will be now described with reference to FIG. 2. When the engine of the ship is started to operate the pump and the water jet is discharged from the nozzle B, the flow of the water in the pipe between the intake port A and the outlet nozzle B has a characteristic shown by the nozzle characteristic curve J in FIG. 2. The operating point moves on the curve J, according to the speed of the ship, from the origin (Q=0, H=0) shown in FIG. 2 which indicates a state of the stoppage of the ship, to the direction, as shown by an arrow in FIG. 2.
Hereupon considering the pressure (water head) H.sub.1 at the point C immediately before the nozzle B when the engine (or the turbo pump) is driven at a prescribed number of revolution and the ship is restrained from moving, it is indicated by a characteristic curve H.sub.1 dependent on the inclination of a pump head curve which is provided by subtracting the sum of all the pressure loss head in the pipe between the intake port A and the nozzle B from the head curve of the turbo pump. The intersecting point Q.sub.1 of the characteristic curve H.sub.1 with the nozzle characteristic curve J provides the point of operation of the water jet. When the ship is then freed from the restraint and travelled, dynamic pressure resulting from the speed of the ship acts on the intake port A so that the pressure head H.sub.1 at the point C immediately before the nozzle B moves, on the nozzle characteristic curve J, from the point Q.sub.1 to a point Q.sub.1 ' at which a thrust corresponding to the resistance to the travel of the ship is produced, whereby the characteristic curve H.sub.1 rises to that of H.sub.1 '. The pressure head of the share of such rise from the level of the characteristic curve H.sub.1 to that of the upper one H.sub.1 ' corresponds to the dynamic pressure V.sub.s.sup.2 /2g wherein V.sub.s and g denote the speed of the ship and the acceleration of gravity, respectively. Accordingly, the pressure H.sub.1 ' at the point C immediately before the nozzle C during the travel of the ship can be calculated in accordance with a following equation (1). ##EQU1## wherein H: head of the pump (m), h.sub.L : sum of many kinds of pressure losses of head such as the friction loss in the pipe between the inlet A and the nozzle B (m),
V.sub.s : speed of the ship (m/s).
Although the design of a pump must be effected by using the flow rate Q.sub.n at the point Q.sub.1 ', it is mostly designed by using the flow rate Q.sub.n ' at a point P or a point P', which is smaller than Q.sub.n, because it is difficult to presume the resistance to the hull of the ship when it is travelling and estimate the pressure loss of head in the pipe between the inlet A and the nozzle B. If the turbo pump having the flat inclination of the pump head curve used in the jet propulsion system is designed by the use of the flow rate Q.sub.n ', the characteristic curve of the pressure head H.sub.2 immediately before the nozzle B is flatter in inclinatin than that of the pressure head H.sub.1. In that case, the point of operation when the ship is travelling is denoted by Q.sub.2 ' as shown in FIG. 2.
The thrust T when the ship is travelling is calculated in accordance with a following equation. EQU T=.rho..multidot.Q.sub.P (V.sub.j -V.sub.s) (2)
wherein T: Thrust (kgf),
.rho.: Density of the water (kgf.s.sup.2 /m.sup.4), PA0 V.sub.j : Jet speed (m/s), PA0 Q.sub.p : Flow rate of the pump at the point P (m.sup.3 /s).
The jet speed V.sub.j is calculated in accordance with a following equation. ##EQU2## wherein H.sub.sv : Effective recovered dynamic pressure ##EQU3##
As apparent from the equations (2) and (3), it is understood that the thrust T is proportional to the flow rate of the turbo pump and increases nearly in proportion to the square root of the pump pressure. This means that the more the nozzle characteristic J advances to the direction of arrow-mark, the more the thrust increases. Accordingly, the flatter the head characteristic of the pump used for the jet propulsion system is, the more the intersecting point with the nozzle characteristic J moves toward the side greater in flow rate Q and the thrust is increased, thereby permitting increase in the speed of the ship. Conversely speaking, such pump may be also applied to a widely used hull of greater resistance and an estimated margin for the resistance to the hull of the ship, the pressure loss in pipe line and so forth can be made greater, with the result that the allowable tolerances in the design of the water jet propulsion system are in a wide range.
In the water jet propulsion system, it is on the other hand necessary for improvement of the propulsion efficiency that the turbo pump is directly coupled to the engine to make the speed of the pump as high as possible, and a reduction gear or the like between the engine and the pump is eliminated to reduce the size and weight of the entire propulsion system. However, since the pump head is proportional to the square of the number of revolution rather than to the law of pump similarity, the higher the speed of the pump is made, the farther the pump is apart from the aformentioned requirement of flattening the head curve of the pump. Besides, the jet propulsion system requires a high-specific-speed pump of higher flow rate and lower head. Since the head characteristic of such high-specific-speed pump has a greater inclination going down to the right than some of the turbo pumps, the higher speed of the pump results in the characteristic curve having an extremely greater slope going down to the right, so that the higher speed of the pump cannot be achieved. Moreover, even if such general high-specific-speed pump is designed to be rotated at a lower speed, the pump has generally the following features and undesirable properties for the water jet propulsion system.
(1) The slope of the head curve of the pump going down to the right is large.
(2) The efficiency of the pump is considerably low if it is driven away from the point of the maximum efficiency thereof.
(3) At the excessive flow rate beyond the maximum efficiency of the pump, cavitation is likely to occur, thereby resulting in the sharp drop of the efficiency.